xe2x80x9cThis application is a 371 application of PCT/JP00/00495 filed Jan. 31, 2000.xe2x80x9d
The present invention relates to a production method of a threo-1,2-epoxy-3-amino-4-phenylbutane derivative represented by the following general formula (1): 
wherein P represents a urethane-type amino-protecting group and the configurations at 2 and 3 positions are (2S,3R) or (2R,3S). The threo-1,2-epoxy-3-amino-4-phenylbutane derivatives (1), in particular (2R,3S)-1,2-epoxy-3-amino-4-phenylbutane derivatives, are compounds useful as intermediates for the production of various HIV proteases, as described in Japanese Kokai Publication Hei-05-230095.
The hitherto-known production technology for threo-1,2-epoxy-3-amino-4-phenylbutane derivatives (1), in particular (2R,3S)-1,2-epoxy-3-amino-4-phenylbutane derivatives, includes a method which comprises synthesizing (2R,3S)-1,2-epoxy-3-N-(tert-butoxycarbonyl)amino-4-phenylbutane by epoxidizing 2(S)-N-(tert-butoxycarbonyl)amino-1-phenyl-3-butene with m-chloroperbenzoic acid in methylene chloride (e.g. J. Org. Chem., 1987, vol. 52, page 1487; Japanese Kokai Publication Hei-05-230095). Further, there is also known the method for isolating (2R,3S)-1,2-epoxy-3-amino-4-phenylbutane derivatives, which comprises carrying out ether extraction and washing repeatedly, drying the organic phase over anhydrous sodium sulfate, evaporating the solvent, purifying the residue by silica gel column chromatography (eluent: hexane/ethyl acetate=4/1) and recrystallizing the product from hexane (Japanese Kokai Publication Hei-05-230095).
However, the prior art methods pose serious problems in carrying out them on an industrial scale; for example, the reaction yield of (2R,3S)-1,2-epoxy-3-N-(tert-butoxycarbonyl)amino-4-phenylbutane is low, namely about 80 mole percent (J. Org. Chem., 1987, vol. 52, page 1487); it is necessary to use undesirable reagents (e.g. methylene chloride, which is a halogenated hydrocarbon, ether and hexane, which are very inflammable organic solvents, and m-chloroperbenzoic acid, which is a very hazardous peroxide, or the like) ; a plurality of organic solvents are used in large quantities; the procedure is complicated; and the productivity is low.
In the prior art methods, silica gel column chromatography (eluent: hexane/ethyl acetate=4/1) and/or recrystallization from hexane is carried out for purification and isolation of (2R,3S)-1,2-epoxy-3-N-(tert-butoxycarbonyl)amino-4-phenylbutane. Such an isolation procedure, however, involves serious problems in carrying it out on a commercial scale; for example, use of large amounts of undesirable reagents, complexity of the process, a waste of time resulting from the complicatedness, increases in the number and capacity of production units, and low yields. A further problem is that the product crystallized from hexane has a low density, hence a large capacity container is required for packaging crystals of the above compound.
In particular, investigations made by the present inventors revealed that the affinity of (2R,3S)-1,2-epoxy-3-N-(tert-butoxycarbonyl)amino-4-phenylbutane and other threo-1,2-epoxy-3-amino-4-phenylbutane derivatives (1) with various organic solvents is not suitable, which makes it very difficult to isolate high-quality crystals in good yield by crystallization at an appropriate crystallization concentration.
Thus, there is no effective method available in the art for producing a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1), in particular (2R,3S)-1,2-epoxy-3-N-(tert-butoxycarbonyl)amino-4-phenylbutane.
The above threo-1,2-epoxy-3-amino-4-phenylbutane derivatives (1) are intermediates for the production of HIV protease inhibitors which need to be ingested in high doses, and, therefore, it is of particular significance to develop a practical method for mass production of said derivatives.
In view of the above state of the art, it is an object of the present invention to provide a production method of high quality threo-1,2-epoxy-3-amino-4-phenylbutane derivatives (1) on a commercial scale in a simple, easy and efficient manner and with very high productivity. Another object of the invention is to develop a method of crystallizing the above derivatives in a high density state.
The present invention thus provides a production method of a threo-1,2-epoxy-3-amino-4-phenylbutane derivative of the above general formula (1)
which comprises treating, with a base, a threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative of the following general formula (2): 
wherein P is as defined above, X represents a halogen atom and the configurations at positions 2 and 3 are (2S,3R) when they are (2S,3R) in the formula (1) or (2R,3S) when they are (2R,3S) in the formula (1), in a polar organic solvent or a solvent composed of a polar organic solvent and water,
and adding the resulting reaction mixture to water to thereby cause the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) to crystallize out.
The present invention further relates to a synthesis method of a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1)
which comprises treating, with a base, a threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) in a polar organic solvent or a solvent composed of a polar organic solvent and water.
The present invention further relates to a purification/isolation method of a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1)
which comprises adding, to water, a solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a polar organic solvent or a solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a solvent composed of a polar organic solvent and water to thereby cause the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) to crystallize out.
In the following, the present invention is described in detail.
The threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1), which is the desired compound to be produced by the method of the present invention, is a protected form of threo-1,2-epoxy-3-amino-4-phenylbutane having the amino group protected by a urethane type protective group.
In the above general formula (1), the group P attached to the amino group is an amino-protecting group. This amino-protecting group is a group capable of protecting an amino group and includes various routine protective groups such as those described in the relevant monographs or literature available in the art, for example xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d, 2nd edition, John Wiley and Sons, 1991. As the protective group P in the above general formula (1), preferred is a urethane type protective group (also referred to as carbamate type protective group). Preferred as such group are, among others, aralkyloxycarbonyl groups and lower alkoxycarbonyl groups (the alkyl moiety having 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms). More preferred are benzyloxycarbonyl, methoxycarbonyl, ethoxycarbonyl and tert-butoxycarbonyl. Particularly preferred is tert-butoxycarbonyl.
The configurations at positions 2 and 3 of the above threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) are either (2S,3R) or (2R,3S).
The threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative of the above general formula (2), which serves as the substrate in the reaction involved in the production method according to the present invention, has the same configuration as the above threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1). Thus, when the above threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) , which is to be obtained, has the (2S,3R) configuration, the above threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) should have the (2S,3R) configuration. For obtaining the (2R,3S) product, the substrate should be of (2R,3S) configuration.
In the above general formula (2), the group P attached to the amino group is as mentioned above. X represents a halogen atom, such as a chlorine, bromine, fluorine or iodine atom. A chlorine or bromine atom is preferred and a chlorine atom is more preferred because of ease of substrate synthesis.
The above threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) can be prepared, for example, by reducing a 1-halo-2-oxo-3-amino-4-phenylbutane derivative of the following general formula (3): 
wherein P and X are as defined above and the configuration at position 3 is R when the threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) is intended to have the (2S,3R) configuration, or S when the derivative (2) is intended to have the (2R,3S) configuration. More specifically, the (2R,3S) form of a threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) can be obtained by microbiological reduction of the corresponding (S) form 1-halo-2-oxo-3-amino-4-phenylbutane derivative (3), as described in Japanese Kokai Publication Hei-09-285.
The above 1-halo-2-oxo-3-amino-4-phenylbutane derivative (3) can be prepared by the methods described in Japanese Kokai Publication Sho-62-126158 and Japanese Kokai Publication Hei-02-42048, among others.
In cases where the above threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) is derived from the above 1-halo-2-oxo-3-amino-4-phenylbutane derivative (3) by a reduction reaction, there is a tendency for the threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) to contain the 1-halo-2-oxo-3-amino-4-phenylbutane derivative (3) as an impurity. However, even such threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) can be properly used as the reaction substrate to give the corresponding threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) of high quality in high yield. In this respect, the production method according to the present invention is highly advantageous.
According to the production method of the present invention, the threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) is treated, in the first step, with a base in a polar organic solvent or a solvent composed of a polar organic solvent and water to give the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1).
The above-mentioned polar organic solvent is not particularly restricted but includes, among others, aprotic polar organic solvents such as acetone, methyl ethyl ketone, tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, 1,2-diethoxyethane, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, polyethylene glycol diethyl ether, acetonitrile, dimethylformamide and dimethyl sulfoxide; protic polar organic solvents such as alcohols, for example methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol and tert-butanol. These may be used singly, or two or more of them may be used in combination. The solvent may contain any other type of organic solvent if it will not produce any adverse effect.
When an alcohol is used as the above polar organic solvent, there is a tendency toward impurity formation. Therefore, in the treatment with a base, an aprotic polar organic solvent is preferably used. Ketones, such as acetone, ethers, such as tetrahydrofuran, acetonitrile and the like are preferred among others. Acetone is most preferred because of ease of handling and inexpensiveness, however.
In cases where a solvent composed of a polar organic solvent and water is used, the polar organic solvent is preferably selected from among organic solvents having relatively high affinity with water, more preferably from among organic solvents having highly affinity with water, still more preferably from among organic solvents fully miscible with water. The term xe2x80x9corganic solvent fully miscible with waterxe2x80x9d generally means an organic solvent such that, when it is stirred gently with the same volume of pure water at a temperature of 20xc2x0 C. and under atmospheric pressure, the resulting mixture, after flows have subsided, has a homogeneous appearance.
The solvent composed of a polar organic solvent and water preferably has a low water content from the standpoint of increasing the reaction rate but, from the standpoint of the ease of crystallization of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) after the reaction, a lower polar organic solvent content is preferred. The water/polar organic solvent ratio may vary according to such factors as the polar organic solvent species, strength of the base, and reaction temperature and therefore cannot be specified in general terms. The ratio, however, can be selected by carrying out a simple experiment. Generally, the ratio is not more than 10 by volume, preferably not more than 5, more preferably not more than 2.
The substrate concentration in the reaction mixture is not particularly restricted but, generally, the ratio of the threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) to the reaction solvent is not less than 5 w/v %, preferably not less than 10 w/v %.
The base to be used in the production method according to the invention is not particularly restricted but includes, among others, alkali metal hydroxides, alkali metal carbonates, alkaline earth metal hydroxides and alkaline metal carbonates. These may be used singly or two or more of them may be used combinedly. From the reaction rate viewpoint, alkali metal hydroxides and alkaline earth hydroxides are preferred and, from the viewpoint of inexpensiveness, ease of handling and/or ease of waste water treatment, for instance, alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide are preferred. Among them, sodium hydroxide is particularly preferred.
The above base may be used in the solid form or in the form of an aqueous solution or suspension. From the operability viewpoint, however, it is preferably used as an aqueous solution. For example, a 1 to 20 N aqueous solution of an alkali metal hydroxide is favorably used.
The above base is used approximately in a stoichiometric amount or larger. The use of an excessively large amount is uneconomical. Generally, the base can be used in an amount of about 1 to 10 equivalents, preferably about 1 to 3 equivalents.
The reaction temperature in the step of treatment with a base is not particularly restricted. Thus, the reaction can be carried out at a temperature at which the reaction mixture will not solidify through at a temperature not higher than 50xc2x0 C., preferably at a temperature not higher than about 30xc2x0 C.
The reaction time for said step of treatment with a base is not particularly restricted. When the base is used in the above-mentioned amount and the reaction temperature is within the above range, it is possible to carry the reaction to complete generally in several minutes to 30 hours. When the base is used in an amount of about 1 to 3 equivalents and the reaction temperature is about 10xc2x0 C. to about 30xc2x0 C., the reaction can be carried to complete generally within several hours.
The second step in the production method according to the present invention is carried out by adding, to water, the reaction mixture obtained in the above manner to thereby cause the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) to crystallize out.
In this crystallization step, the presence of water as a poor solvent is essential for properly reducing the solubility of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1). However, the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) tends to separate out as an oil (oily substance) in the presence of water. For example, when water is continuously added to the reaction mixture obtained, the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) separates out as an oil (oily substance) and this oil (oily substance) solidifies and adheres to the crystallizer wall and/or stirrer in substantial amounts, resulting in deposition of tight scales and/or formation of hardly disintegrable lumps. As a result, unfavorable phenomena occur; for example, the crystallizer contents cannot be recovered neatly from the crystallizer. Therefore, the method of adding the reaction mixture to water is required so that nucleation may be promoted in highly water-rich state while such oil formation is prevented.
For successful crystallization, the reaction mixture obtained is added to water preferably over a period of half an hour or longer, more preferably over about 1 hour or longer, more preferably gradually over a still more extended period of time. Although there is no particular upper limit to the period of addition, the period of addition is generally not longer than about a day, preferably not longer than about half a day.
A procedure for maturation may be incorporated by interrupting the addition of the reaction mixture to water during the addition. This procedure is helpful in causing the crystallization to proceed while appropriately solidifying the oil (oily substance) accumulated with the addition of the reaction mixture to water. Although the timing of interposition of said maturation procedure is not particularly restricted, it is particularly effective to interpose the above maturation procedure at an early stage of addition, for example after addition of 1/1,000 to 1/10, preferably 1/1,00 to 1/20, of the reaction mixture obtained. The duration of such maturation treatment is preferably at least half an hour, more preferably about 1 hour or longer. After completion of the above maturation procedure, the addition of the reaction mixture obtained can of course be resumed for continued crystallization. If necessary, the procedures for addition and maturation may be repeated alternately.
By following the above procedure, it is possible to obtain a good crystal-containing slurry while suppressing the above-mentioned phenomenon inducedby solidification of the oil (oily substance).
The temperature for the crystallization step may depend on the species of threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1), the polar organic solvent which coexists, the ratio between water and the polar organic solvent and other factors. For suppression of oil formation and promotion of crystallization, however, a low temperature is desirable. Generally, the crystallization is carried out preferably at a temperature at which the liquid will not solidify through at a temperature not higher than 40xc2x0 C., more preferably not higher than about 30xc2x0 C., still more preferably not higher than about 20 C., most preferably not higher than about 10xc2x0 C. For improving the crystal characteristics, quality and yield, it is also possible, if necessary, to conduct heating and/or cooling after crystallization.
The ratio between water and the polar organic solvent in the crystallization solution is preferably selected so that the crystallization solution will not separate into two or more phases but the crystallization may proceed in a solution forming a homogeneous phase between water and the polar organic solvent. Such ratio cannot be specified in general terms since it depends on the species of threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1), the polar organic solvent used, the crystallization temperature and the intended extent of impurity removal and other factors. However, the ratio can be easily selected by carrying out a simple experiment. When the polar organic solvent used has a high affinity for the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1), for instance, it is desirable to increase the proportion of water.
Generally, from the viewpoint of satisfactory crystallization and improved percent recovery of the threo-1,2-eopxy-3-amino-4-phenylbutane derivative (1), a high proportion of water is preferred. The weight ratio of the polar organic solvent/water in the crystallization sytem is thus preferably not more than 1, more preferably not more than about xc2xd, still more preferably not more than about xc2xc. If necessary, for the purpose of promoting the crystallization and improving the recovery rate, the proportion of the polar organic solvent can be reduced by distilling off the polar organic solvent out of the system under reduced pressure, for instance, during and/or after the crystallization step.
The concentration of the threo-1,2-eopxy-3-amino-4-phenylbutane derivative (1) in the crystallization mixture is not particularly restricted but the crystallization can be carried out at a concentration of not lower than about 5% by weight.
As regards the intensity of stirring or degree of agitation in the step of crystallization, excessively weak stirring tends to result in insufficient dispersion, while excessively vigorous stirring results in formation of too fine crystals, hence formation of a frothy slurry. For obtaining crystals having satisfactory properties and uniform particle size, therefore, moderate stirring or moderate agitation is preferred.
In the crystallization step, the crystallization solution may contain some organic solvent other than the solvent derived from the above-mentioned reaction step, and/or an inorganic salt such as sodium chloride, each in a concentration at which no adverse effect is produced.
The above technology will be described in further detail, referring to a specific procedure.
In carrying out the crystallization, the base which may remain in the reaction mixture can be removed after completion of the reaction or in the crystallization step. A typical procedure for removing the base comprises neutralization with an acid. The acid to be used for neutralization is not particularly restricted but may be a mineral acid such as hydrochloric acid, sulfuric acid and phosphoric acid, an organic acid such as acetic acid and formic acid, and a salt thereof. Such acids may be used singly or in combination. Among such acids, mineral acids such as hydrochloric acid and sulfuric acid are preferred because of low cost, ease of handling and ease of waste water treatment, among others. A preferred combination of such an acid with the base in the practice of the present invention is a combination such that, upon neutralization, they form an inorganic salt facilitating waste water treatment.
An alternative method for removal of the base, which applies when the reaction mixture separates into two phases (an organic phase and an aqueous phase), comprises removing the aqueous phase containing a large amount of the base out of the system.
Another procedure for removing the base, which applies when the base has precipitated out from the reaction mixture, comprises filtering off the base.
For preventing the basic component and inorganic salt from contaminating the crystals as completely as possible, the reaction mixture after the removal of the base component by the above-mentioned procedure should be submitted to crystallization and the base still remaining in the crystallization solution be neutralized. The target degree of neutralization is such that the pH in the crystallization step will be 6 to 11, preferably 7 to 9. For preventing contamination of the crystals with the base component and inorganic salt, it is also effective to thoroughly wash the wet crystal crop with water.
For promoting crystallization (nucleation), it is preferred that seed crystals are added in carrying out the crystallization. The addition amount of seed crystals depends on the rate of addition of the reaction mixture obtained and other factors, hence cannot be specified in general terms. The preferred amount is at least 1% by weight relative to the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) subjected to crystallization.
The thus-obtained crystals of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) can be recovered by an ordinary solid-liquid separation technique such as pressure filtration, filtration under reduced pressure or centrifugation and, after washing the wet cake with water containing a small amount of an organic solvent and/or with water, if necessary, they can be dried by drying under atmospheric pressure, fluidized-bed drying or vacuum drying, for instance.
According to the production method of the present invention, it is possible to produce a high-quality threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) very efficiently in a simple and easy manner. Generally, the yield based on the corresponding threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) can be expected to be not less than about 90 mole percent. The crystals of said compound as obtained can be expected to have a higher density as compared with the crystals obtained by the prior art methods, for example a bulk density upon loose packing of not less than 0.3 g/ml, preferably not less than 0.4 g/ml.
The method of synthesizing a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) according to the second aspect of the present invention comprises the first step of the above-mentioned production method. Thus, it comprises treating a threo-1-halo-2-hydroxy-3-amino-4-phenylbutane derivative (2) with a base in a polar organic solvent or a solvent composed of a polar organic solvent and water. This method of synthesis can be carried out in the same manner as the first step of the above-mentioned method of production. Such method of synthesis can be expected to give the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a reaction yield of not less than about 90 mole percent, preferably not less than about 95 mole percent.
The purification/isolation method according to the third aspect of the present invention comprises crystallizing a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) by adding, to water, a solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a polar organic solvent or a solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a solvent composed of a polar organic solvent and water. By utilizing this purification/isolation method, it is possible to purify and isolate a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) obtained by some reaction step other than the first step of the above-mentioned production method by following the same procedure as in the second step of the above-mentioned production method.
The reaction mixture obtained after reaction can of course be used as the solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a polar organic solvent or the solution of the threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a solvent composed of a polar organic solvent and water. The solution to be used may also be a solution separatedly prepared by dissolving a threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) in a polar organic solvent or a solvent composed of a polar organic solvent and water.
Further, it may be a modification of the reaction mixture obtained in the above manner as rendered more suited for crystallization by concentration or solvent substitution.
The polar organic solvent to be used in the above case is not particularly restricted but includes, among others, aprotic polar organic solvents such as acetone, methyl ethyl ketone, tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, 1,2-diethoxyethane, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, polyethylene glycol diethyl ether, acetonitrile, dimethylformamide and dimethyl sulfoxide; protic polar organic solvents such as alcohols containing 1 to 4 carbon atoms, for example methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol and tert-butanol, and acetic acid. These may be used singly, or two or more of them may be used in combination.
Generally preferred as the above polar organic solvent is an organic solvent relatively high in affinity with water, more preferably an organic solvent high in affinity with water, still more preferably an organic solvent fully miscible with water. The term xe2x80x9corganic solvent fully miscible with waterxe2x80x9d generally means an organic solvent such that, when it is stirred gently with the same volume of pure water at a temperature of 20xc2x0 C. and under atmospheric pressure, the resulting mixture, after flowing has subsided, has a homogeneous appearance. Specifically, ketones such as acetone, lower alcohols (alcohols containing 1 to 4 carbon atoms) such as methanol, and acetonitrile are preferred among others. From the viewpoint of ease of handling, inexpensiveness and oil formation-inhibiting effect, among others, acetone and methanol are particularly preferred. In the step of crystallization, the mixture may contain some other organic solvent and/or an inorganic salt such as sodium chloride each in a concentration at which no adverse effect is produced.
In this purification/isolation method, the conditions other than those just mentioned above are the same as those mentioned above referring to the second step of the above-mentioned production method.
In accordance with such purification/isolation method, the recovery percentage of the high-quality threo-1,2-epoxy-3-amino-4-phenylbutane derivative (1) can be expected to be not less than about 90 mole percent, preferably not less than about 95 mole percent. The density of the crystals of said compound can be expected to be higher as compared with the crystals obtained by the prior art methods, for example a bulk density upon loose packing of not less than 0.3 g/ml, preferably not less than 0.4 g/ml.